The prior art teaches preparation of polytetrafluoroethylene composite sheets having low-level water-absorbency. U.S. Pat. No. 4,153,661 relates to sheets comprising particulate materials distributed in a matrix of interentangled polytetrafluoroethylene fibrils. In these sheets water is absorbed primarily by filling voids between particles, not by swelling of particles. These prior art sheets contain primarily inorganic particulates, although a few organic particulates which have low-level water-absorbency are mentioned. Such organic particulates include the proteins casein and zein, and certain polyurethanes. The patentee discloses the use of 5 to 10 weight percent of water-swellable thermoplastic materials for enhancing the tensile strength of the composites. No composites are described in which all or a major portion of the particulate is highly water absorbent or water-swellable. The composites of the above patent are not known to be useful as wound dressings.
It has been recognized in the prior art that a satisfactory wound dressing creates a suitable microclimate for rapid and effective healing. A good wound dressing prevents dehydration and scab formation, is permeable to oxygen, is sterilizable, absorbs blood and exudate, protects against secondary infection, supplies mechanical protection to the wound, is non-adherent, is non-toxic, is non-allergenic and non-sensitizing, does not shed loose material into the wound, conforms to anatomical contours, resists tearing, resists soiling, is not inflammable, has constant properties in a range of temperatures and humidities encountered in use, has long shelf life, has small bulk, is compatible with medicaments, and is economical.
Layered dressings are known in the art. U.S. Pat. No. 4,203,435 discloses a five-layered wound dressing having two permeable, non-adherent outer layers, the next two layers being cellulosic fiber absorbent layers, and the inner-most layer comprising a powder of modified starch. U.S. Pat. No. 3,888,257 provides a layered disposable, absorbent article for management of body fluid flow, in which a central zone of a matrix of fiberized wood pulp incorporates a three-dimensional dispersion of hydrocolloid polymer particles.
Great Britain Pat. No. 1,454,055 discloses a preparation, such as a bandage, for treating a fluid-discharging skin surface, which comprises a water-insoluble hydrophilic macromolecular swellable material such as crosslinked dextran, carboxymethyldextran, starch or hydroxyethyl starch for absorbance of low molecular weight constituents of blood plasma in admixture with a dermatologically suitable carrier. The swellable material may be dextran, or a derivative thereof, and the carrier may comprise fibrous material. This patent discloses no fibrous materials having submicron diameters.
A crosslinked dextran derivative which is sold by Pharmacia of Sweden under the tradename "Debrisan", is primarily used as a wound treating powder. Pharmacia's trade literature on Debrisan.RTM. asserts the following advantages for the use of the powder: "continuously absorbs wound exudate and bacteria, cleanses the wounds, prevents crust formation, reduces inflammation and oedema and does not cause sensitization". The trade literature which states that no side effects have been reported also gives the following limitations on the product: "(1) do not leave Debrisan.RTM. for more than 24 hours on wounds with a very low exudation rate as it may dry and form a crust which may be difficult to wash off, (2) occlusive dressings may lead to maceration of skin around the wound under treatment, (3) when deep infected wounds are treated, care must be taken to wash Debrisan.RTM. from the depths of the wound, and (4) no side effects have been reported. Warning: Debrisan.RTM. spillage can render surfaces very slippery. Clear spillages promptly." Thus, it appears that although the dextran derivatives have excellent absorptive capacities, there are problems associated with their use relating to moisture loss, adhesion, contamination of the wound, and handling hazards.